In the production of composite cans, several layers of material are spirally wound onto a mandrel in order to form what is referred to as a composite tube. The rotating tube advances down the mandrel as it is wound. As the tube advances, it is severed by cutting means which are generally carried on a reciprocating carriage.
Generally, composite tubes have been severed by means of knives which are intermittently forced against the rotating tube. This type of cutting method has inherent speed limitations which typically require that cutting be done in two steps. Initially, sections of tube containing several individual composite cans are cut off on the same machine where the tube is formed. Each section is then sent to a second machine, which recuts the section into individual can lengths.
In order to overcome the speed limitations of the mechanical cutting method described above, it has been proposed to use a laser beam as a means for cutting the tube. This would allow elimination of the recutting machine. Several U.S. patents disclose using a laser beam to cut material. U.S. Pat. No. 3,604,890, utilizes a laser to cut various materials. U.S. Pat. No. 3,690,744 discloses the use of a laser beam to form a line of weakness in laminated material. The use of lasers to cut off tubular bodies is disclosed in U.S. Pat. Nos. 3,960,624 and 3,679,863. Laser cutting has distinct advantages over the mechanical knife method, notably the cleanliness of the cut and the high attainable cutting speeds.
Because the wrapping angle of spirally wound composite cans is such that the tube rotates less than one revolution in the course of making a can, it is necessary to utilize two laser beams, each having enough power to completely cut through composite material. By utilizing two laser beams, the tube may be completely severed before it advances one can length. This allows time to reposition the laser beams so as to sever the next can from the tube. Each beam would be required to cut through one half of the circumference of a tube. One possible way of accomplishing this is to utilize two separate high power laser sources. For reasons of efficiency and economy, however, it is preferable to employ a single high power laser source and divide it into two separate beams in order to cut the tube.
There are many instances where it is useful to direct two separate laser beams to a workpiece. Often this is accomplished by employing a laser source having a single output beam and dividing it into two components by means of a beamsplitter. This type of use is disclosed in U.S. Pat. No. 3,604,890, where laser beams of different focal length are directed to a single area in order to cut various materials. In U.S. Pat. No. 3,410,203, a beamsplitter divides a laser into two components which are used in a non-impact printer. U.S. Pat. No. 3,778,585 utilizes a beamsplitter to provide two beams which are used to protect the laser source against backscatter damage. In U.S. Pat. No. 3,989,778 a beamsplitter is utilized to divide a laser beam into two components which are then directed to opposite sides of sheets of thermoplastic material which are to be sealed together. In all of the above U.S. Patents the beamsplitter is positioned so that the laser beam strikes it at a 45.degree. angle.
In the cutting of composite tubes it is desirable to split a laser beam into two components which have a constant output and whose power ratio remains a constant in order to achieve greatest efficiency. The difficulty with this is that many lasers have unstable polarization characteristics (i.e. the polarization of the laser beam varies over time) and beamsplitters are polarization dependent at any angle of incidence other than normal. This polarization dependence is greatest at the commonly used angle of incidence of 45.degree.. At angles of incidence near normal the polarization dependence is minimized. In other words, at near normal angles of incidence the degree of reflectivity of a beamsplitter is nearly constant despite varying polarization of the incident laser beam. The present invention relates to the cutting of composite tubes at two distinct points with a system employing only a single laser source.